Compressors in refrigeration systems raise the pressure of a refrigerant from an evaporator pressure to a condenser pressure. The evaporator pressure is sometimes referred to as the suction pressure and the condenser pressure is sometimes referred to as the discharge pressure. Many types of compressors, including rotary screw-type compressors, are used in such refrigeration systems. Rotary screw compressors are positive displacement, volume reduction devices.
A rotary screw-type compressor includes a suction port and a discharge port that open into a working chamber of the compressor. The working chamber includes a pair of meshed male and female screw rotors in a compressor housing that define a compression pocket between the screw rotors and interior walls of the working chamber of the compressor housing. The working chamber of the compressor housing defines a volume shaped as a pair of parallel intersecting flat-ended cylinders, with the each rotor housed primarily in one of the cylindrical volumes.
In conventional operation of refrigeration-based systems, the counter-rotation of the intermeshing screw rotors draws a mass of refrigerant gas at suction pressure into the suction port from a suction area at the low pressure end of the compressor. The refrigerant is delivered through the suction port to a compression pocket having a chevron shape, sometimes called a flute space. The compression pocket is defined by the intermeshed rotors and the interior wall of the working chamber. As the intermeshing screw rotors rotate, the compression pocket is closed off from the suction port. Gas compression occurs as the compression pocket volume decreases as the intermeshing screw rotors rotate. The compression pocket is circumferentially and axially displaced to the high pressure discharge end of the compressor by the rotation of the intermeshing screw rotors and comes into communication with the discharge port. The compressed refrigerant gas is discharged radially and axially through the discharge port from the working chamber.
It is often desirable to operate such screw compressors at part-load conditions, such as when full capacity operation is not required. To improve performance at part-load conditions, several approaches have been employed. One approach that has been employed is the use of slide valve arrangements that control the amount of time the gas is compressed before release into the discharge port. Generally, the longer the gas is maintained in the compression pocket of the rotor, the higher the volume ratio of the inlet port to the outlet port. Slide valves allow the volume ratio to be changed based on conditions of the system, improving efficiency. However, interference of the slide valve with the rotors is desired to be avoided. As a result, complex arrangements have been developed to avoid such interference, which increase cost and maintenance of the compressor and limit the ability to control the compression ratio. Furthermore, when the capacity of the system is changing, changes in the volume ratio can result in diversion of gas back to the suction port of the compressor, causing suction gas heating and requiring re-compression of the diverted gas, reducing efficiencies.
Another approach that has been employed to improve part-load performance is the use of variable speed drives (VSDs). VSDs control motor loading by varying the speed that a motor drives the intermeshing screw rotors. VSDs typically vary the frequency and/or voltage provided to the motor. This frequency or voltage variance can allow the motor to provide a variable output speed and power in response to the load on the motor.
Employing VSDs in conventional screw compressors can cause reduced efficiency at full-load capacity. Another challenge with employing VSDs is that conventional motors reach their peak efficiency at their rated speed. As a result, motor efficiency drops at lower speeds. Such reduced theoretical performance compromises the energy savings level at part-load conditions.
Regardless of which approach is employed to achieve part-load performance, neither slide valve arrangements nor variable speed drives used independently in conventional screw compressors have resulted in variable capacity screw compressors that achieve desired efficiencies and operational control. Therefore, further improvements in methods and systems for operation of rotary compressors are desirable.